Phthalates and many other esters are well-known as plasticizers for PVC. These esters are commonly produced by esterification of an acid (or anhydride) with a primary alcohol. Esters may also be derived from secondary alcohols.
The effectiveness of secondary alcohol esters as plasticizers is known to be dependent on the position of the hydroxyl group in the alcohol molecule. Secondary alcohol esters with a high degree of hydroxyl content at the second carbon are considerably more effective than if the hydroxyl group is located more toward the interior of the carbon chain. The use of secondary alcohol esters as plasticizers has been limited by the availability of secondary alcohols with a high C2 hydroxyl content (high C2-OH). 2-Octanol or capryl alcohol obtained from castor oil, is available in limited quantities. For many years dicapryl phthalate was available as a plasticizer for flexible PVC, however because of the costs of the alcohol, this product is not longer available in commercial quantities. Higher molecular weight secondary alcohols are also of interest in surfactant applications where they have improved properties such as wetting efficiency over surfactants derived from primary alcohols. Secondary higher alcohol alkoxylates are advantaged over comparable primary alcohol alkoxylates by having lower pour points, less foam, and excellent detergency and emulsifying power.
The use of esters of secondary alcohols as plasticizers in various polymeric systems has recently been described in WO 2009-070398 and WO 2009-070399.
Most secondary alcohols are commercially produced today by oxidation of paraffins. This process is not suitable for production of secondary alcohols for plasticizer use since it is non-selective and produces a broad mixture of products with the OH group located all along the hydrocarbon backbone. Selective direct hydration of C5 to C12 olefins to C5 to C12 secondary alcohols is not a viable process for it produces secondary alcohols with minimal C-2 substitution.
One route to secondary alcohols with a high degree of C2-OH is the reaction of an olefin with a carboxylic acid in the presence of certain zeolite catalysts. This reaction must be conducted at ˜200° C./200 psig and gives conversions on the order of 25-30% in 5 hours. Another route to secondary alcohols with high C2-OH content is therefore desired.
There is one literature report of esterification of alkenes with Ce(IV) sulfate to produce carboxylic acid esters. While the selectivity is high, the activity of this catalyst system is too low to be commercially acceptable.
The present inventors have discovered that secondary alcohols, such as in the plasticizer range (C4-C13), with a high degree of alcohol substitution at the 2 position (C2-OH) can be prepared with commercially acceptable selectivity and activity by the reaction of an α-olefin with a carboxylic acid in the presence of certain cerium catalysts other than Ce(IV) sulfate. The resultant alkyl carboxylate, with a high α-methyl alkyl carboxylate content, can be hydrolyzed to yield the secondary alcohol with a high degree of 2-hydroxy isomer, with very little, if any 3-hydroxy or higher structural isomer by-product.